The present invention relates to a solar cell and to a method for the manufacture thereof.
One way in which solar energy can be regenerated is to raise the temperature of a liquid mass. There are fundamentally two basic structures, namely a planar structure and a tubular structure. The planar cells generally are in the form of a box on the bottom of which circulates a liquid to be heated in coils covered with an absorbent material. The upper surface of the cell which is exposed to solar radiation, is provided with a transparent glass separated from the coils by a layer of air. These cells operate according to the greenhouse effect principle. They have an acceptable efficiency for a limited temperature increase. When this temperature rises, the efficiency drops to a significant extent which is partly due to the losses by thermal convection in the air between the upper glass and the coils. It is therefore desirable to produce a vacuum between the upper glass and the absorbent elements. The planar structure is not adapted to this requirement, so that tubular solar cells have been proposed.
The invention relates more particularly to such cells. It is possible to use the heat pipe principle for draining the collected solar energy and this can be constituted by an envelope divided into two parts, the first part which is exposed to solar radiation is covered with an absorbent material and placed under a vacuum and the second part is placed in contact with the element to be heated. The envelope contains a liquid which vapourizes in the first part, absorbing heat and condenses in the second part, thus giving off the heat which has previously been stored. The cell also has means for supplying the fluid in the liquid phase to the first part of the tube. The absorbent material is generally a metal oxide or an absorbent plastic material.
Many structures have been proposed in the prior art. According to one of these, the absorbent is formed by depositing a thin layer of an oxidized metal on the inner glass envelope. The thermal contact between the absorbent material and the heat pipe is of good quality. However, the metal and the glass have very different expansion coefficients and their mechanical behaviour is detrimentally effected. The coating can tend to become detached.
According to a second method, the heat pipe is made from metal. This leads to the problem of the glass-metal joint between the outer envelope and the inner envelope. This joint ensures the sealing between a medium atmospheric pressure and the space between the two envelopes in which a high vacuum has been formed.
Other solutions have been proposed and, whilst retaining the two inner and outer glass envelopes, it has been proposed to add supplementary metallic elements as the absorbent.
In all these solutions, the thermal contact is not optimum and essentially takes place by tangential contact in accordance with a generatrix of the internal envelope and optionally by reradiation.
For certain applications, it is preferred to use structures other than those described hereinbefore, which do not involve the heat pipe principle and more particularly a structure in which a liquid to be heated circulated in a metal pipe, which has bent back on itself like a hairpin or U arranged within the inner envelope. In this case, it is necessary to ensure a draining of the heat collected by the absorbent to the liquid circulating in the tube. In more general terms, it is necessary to obtain a homogeneous temperature within the inner envelope of the solar cell.